Ruthenium-Catalyzed α-Regioselective Hydroboration of Allenes.

Hydroboration of allenes is powerful and atom-economic approach to the synthesis of organoboranes, such as the highly versatile allylboranes. However, regarding regiocontrol, existing methods uniformly deliver the boron functionality to the less hindered β- or γ-position, but not the α-position. The latter is particularly challenging for allenes with substantial steric difference between the two terminals and lacking electronic bias (e.g., 1,1-disubstituted allenes). Herein we report the first highly efficient ruthenium-catalyzed hydroboration of allenes featuring exclusive α-regioselectivity, providing access to sterically hindered allyl boranes that are limitedly accessible by conventional methods. DFT studies suggested that the unusual α-regioselectivity is attributed to the disfavored reductive elimination at the γ-position due to the high energy cost required to overcome the agostic interaction and rotation of the key π-allyl intermediates. This protocol is also applicable to the previously unprecedented α-hydroalkynylation and underdeveloped α-hydrosilylation of allenes, thus complementing known catalytic systems and providing convenient access to highly congested yet densely-functionalized allyl silanes and skipped enynes bearing a fully-substituted allylic carbon center. It is expected that this ruthenium-catalyzed system can serve as a new platform for the development of other hydrofunctionalization processes with unorthodox selectivity.